I don't think you can say that there's any corporate tax rate where a further reduction wouldn't encourage additional investment. What I think is that there are some countries where changing the corporate tax rate doesn't make much difference, but those are countries with very weak tax administration and very poor general conditions for investment. But for a country like Canada, I couldn't see that there's some rate below which you shouldn't go.
On April 14th, 2008. See this statement in context.